In the truck-tractor industry, the number of axles required on the ground is determined by the load carried by the vehicle. When lighter weight tractors are towed, often only one rear axle is required to accommodate the load (the “primary axle”). As the load weight increases, however, an additional axle may be required to carry the added weight (the “auxiliary axle”). The primary axle is connected to the engine to provide the driving force. The auxiliary axle, when in use, only provides load distribution to the rolling surface and does not provide any driving force.
On trucks with two rear axles, it has been demonstrated that if the load can be carried by one axle, fuel consumption is significantly reduced if the auxiliary axle is lifted so as to not contact the rolling surface. This lifting is typically accomplished through the use of air springs (referred to herein as “lift springs”) and leverage to pivot the auxiliary axle off of the rolling surface. Trucks with this configuration are commonly referred to as 6×2 systems (six wheel-ends with two drive wheel-ends) Systems designed to lift the auxiliary axle off of the rolling surface are well known in the prior art.
Tractors are commonly fitted with a suspension system that adjusts to the weight being carried through the use of air springs (referred to herein as “ride springs”). These springs are filled with pressurized air from the truck's compressor. As the trailer load increases, the air pressure in the ride springs is increased to maintain the proper position of the frame relative to the vehicle's axles. The adjustment of the pressure in the ride springs is accomplished by automatic systems. Through the function of the suspension system, the auxiliary axle and primary axle may not carry the same amount of load.
Federal vehicle safety standards require that all wheels on these trucks be provided with anti-lock brake systems. These brake systems monitor wheel motion during braking to limit wheel lock-up and improve control and stopping distance. The system has a relay valve which receives a braking signal from the driver as a low volume pneumatic pressure and then relays that signal pressure to the service brake chamber with a high volume flow to fill the chamber quickly at the requested pressure. When anti-lock brake system braking systems are used with an auxiliary axle, certain problems can occur.
Specifically, when the auxiliary axle is lifted, undesirable “false events” may be recorded as lock-up by the anti-lock brake system if the system detects that the lifted wheels are not turning during a brake application. To avoid this, separate anti-lock brake system controllers can be used with the lift axle, but the application signal pressure from the vehicle's primary service brake system must be turned off when the auxiliary axle is in the lifted position. When the auxiliary axle is lowered to carry the load, the service application signal must be turned back on to enable the brakes on the wheels attached to the auxiliary axle. Inversion valves, which can be used to shut off pneumatic output based on pneumatic pressure at another location, are known in the art for use to disable the anti-lock brake system in the auxiliary axle when it is in the lifted position, but these are separate devices that are applied to each line.
Additionally, when the auxiliary axle is not lifted, the auxiliary axle and the primary axle may not carry the same amount of load. This causes the contact pressure between the tires and the rolling surface to be different on the primary and auxiliary axles. Because the driver of the truck only has the ability to apply one control pressure, both the axles receive the same braking signal. If the load on the auxiliary axle is lower than that on the primary axle, the wheels on the auxiliary axle may lock-up, generating an anti-lock brake system event. This locking of the wheels can result in the tires being dragged along the road surface, causing wear and flat spots which ruin the tires, which results in added replacement and disposal expense.
There are currently devices known as load sensing valves that adjust output relative to the input based on preset mechanical and pneumatic adjustments, but these adjustments must be made specific to the installation, require complicated calculations, and are subject to installation errors. Accordingly, there is a need in the art for improvements in such devices, and these are now provided by the present invention.